Conventional direct current (DC) ion sources, also called high current density ion sources, are often used in semiconductor processing and in other applications. A typical DC ion source may comprise a plasma system that produces the desired ions and an extraction and focusing system. Plasma may be produced in a plasma chamber by ionization of gas under a high DC or RF electric field. The extraction system uses high voltage to extract an ion beam from the plasma. The extracted ion beam is then focused or formed into a parallel beam by the focusing system. A DC electric field is often used to draw ions in the plasma from the extraction system to the focusing system.
High current density ion sources generally are used in etching applications, ion implantation, and in accelerator technology. In order to provide a high current density ion beam, it is necessary for gas in a plasma system to be of a sufficiently high density in order to provide the high-density conduction charge carriers (electrons and ions). Usually the high-density plasma is created ionizing gas with a high-energy external RF field. However, the plasma density will eventually reach saturation regardless of the strength external RF field.
Most high current density ion sources also use magnetic fields to excite and maintain the plasma. However, the main purpose of the magnetic fields is to confine the plasma flow within the system such that the plasma does not come into contact with the internal surface of the plasma chamber. Magnetic fields force the conduction charge carriers (electron and ions) in the plasma into a circular orbit to reduce the amount of the plasma which otherwise would come in contact with the internal surface of the plasma chamber. The magnetic fields also reduce the need for cooling of the chamber and prevent contamination within the plasma. However, the magnetic fields may also cause the temperature of the plasma itself to increase.
The high-energy external RF field used to ionize and produce the high-density plasma and the magnetic fields produce heat within the plasma system. As the temperature of the plasma system increases, the efficiency of the system decreases. Therefore, a conventional ion source typically uses a cooling system, such as an air or water cooling system.
In addition to receiving energy from of the high energy external field which ionizes the gas, the ions within the plasma also gain energy from the high-energy extraction field produced by an extraction system. The extraction field also causes the conduction charge carriers to move into spiral around the magnetic field line of the extraction field. Thus, the ions in the beam will have a high kinetic energy spread due to both the high electric fields mentioned above and the lateral velocity spread due to the magnetic field, making it difficult for subsequent focusing.
In summary, the following characteristics are found in many currently available high current density ion sources. First, external RF fields and magnetic fields are used to ionize the gas and confine the plasma. Next, most high current density ion sources include a cooling system to dissipate the temperature of the system. Finally, an extraction system comprising a high DC electric field is used to extract the ions from the plasma to produce the ion beam.
A typical ion source is an Electron Cyclotron Resonance (ECR) ion source 100 as shown in FIG. 1. The ECR ion source 100 necessarily operates with a high magnetic field to fulfill the resonance condition of the microwave frequency and electron cyclotron frequency. The microwave power 10 enters a cavity of a plasma chamber 16 through a cylindrical wave-guide 13 and ceramic window 11 to ionize gas that is input via a gas inlet 15. The standard microwave frequency of 2.45 GHz is used, leading to a required magnetic field of 0.0875 tesla to confine the plasma within the cavity 16. In addition, a high DC electric potential is used to extract an ion beam 14 from the plasma. Hence the ion beam 14 has a large energy spread. In addition, in order to produce a plasma with the ECR ion source 100, it is necessary to provide a long collision free path, which limits the pressure in the plasma chamber 16 to be <10-3 mbar. At this low pressure, the ion current density is limited by the available number of gas atoms per cubic centimeter (cm3) in the plasma source. Hence the ECR ion source 100 of FIG. 1 cannot provide high ion beam intensity.
U.S. Patent Publication 20020000779 describes methods for producing a linear array of streaming flux of plasma with low energy ions and electrons to synthesize atomic thin crystal-like thin films on the surface of a substrate. The plasma flux described in this patent publication is suitable for a large cross section area deposition process that does not necessarily need a very high density plasma source.